A typical flooded lead-acid battery includes positive and negative electrode grids and an electrolyte. The electrode grids, while primarily constructed of lead, are often alloyed with antimony, calcium, or tin to improve their mechanical characteristics. Antimony is generally a preferred alloying material for deep discharge batteries.
In a flooded lead-acid battery, positive and negative active material pastes are coated on the positive and negative electrode grids, respectively, forming positive and negative plates. The positive and negative active material pastes generally comprise lead oxide (PbO or lead (II) oxide.) The electrolyte typically includes an aqueous acid solution, most commonly sulfuric acid. Once the battery is assembled, the battery undergoes a formation step in which a charge is applied to the battery in order to convert the lead oxide of the positive plates to lead dioxide (PbO2 or lead (IV) oxide) and the lead oxide of the negative plates to lead.
After the formation step, a battery may be repeatedly discharged and charged in operation. During battery discharge, the positive and negative active materials react with the sulfuric acid of the electrolyte to form lead (II) sulfate (PbSO4). By the reaction of the sulfuric acid with the positive and negative active materials, a portion of the sulfuric acid of the electrolyte is consumed. However, the sulfuric acid returns to the electrolyte upon battery charging. The reaction of the positive and negative active materials with the sulfuric acid of the electrolyte during discharge may be represented by the following formulae.
Reaction at the negative electrode:Pb (s)+SO42− (aq)PbSO4 (s)+2e−
Reaction at the positive electrode:PbO2 (s)+SO42− (aq)+4H++2e−PbSO4 (s)+2(H2O) (l)As shown by these formulae, during discharge, electrical energy is generated, making the flooded lead-acid battery a suitable power source for many applications. For example, flooded lead-acid batteries may be used as power sources for, electric vehicles such as forklifts, golf cars, electric cars, and hybrid cars. Flooded lead-acid batteries are also used for emergency or standby power supplies, or to store power generated by photovoltaic systems.
To charge a flooded lead-acid battery, the discharge reaction is reversed by applying a voltage from a charging source. During charging, the lead sulfate reacts with oxygen molecules from ionized water to produce lead and lead dioxide. The lead dioxide is deposited on the positive electrode and the lead is deposited on the negative electrode.
The lead dioxide deposited on the positive electrode is known to exist in two different crystalline structures, α-PbO2 and β-PbO2. Of these, α-PbO2 tends to have a larger size crystal providing lower surface area over β-PbO2 which has a smaller size crystal with higher surface area. In batteries, the larger crystal size and lower surface area of α-PbO2 tends to lower initial battery capacity, but provides longer life compared to the smaller crystal size and larger surface area of β-PbO2 which tends to provide higher initial battery capacity, but shorter battery life. Upon initial formation, the positive active material paste of a typical deep discharge flooded lead-acid battery tends to exhibit an α-PbO2 to β-PbO2 ratio of about 1.2 or higher.
The addition of tin sulfate to positive electrode in flooded batteries is known, but is generally limited to use in batteries that use positive electrodes made of lead-calcium alloys. Batteries using lead-calcium alloy positive electrode grids are known to suffer from the development of a poorly defined corrosion layer at the surface of the grid which can limit battery life. It has been suggested that the addition of tin to the alloy from which the grid is made, or the application of tin to the surface of a grid will form a tin-enriched layer at the surface of the grid, and in doing so, will improve the properties of the corrosion layer at the interface surface of the grid. However, it is also generally recognized that positive electrode grids of a lead-antimony alloy have well defined corrosion layers, and therefore, would not benefit from the formation of a tin-enriched layer. Moreover, it is also generally recognized that any tin provided at the positive electrode grid will tend to migrate to the negative electrode grid during battery use, and in doing so, will change the half potential of the negative electrode grid and adversely affect the recharge characteristics of the battery.